Electrical resistance element



Aug. 10, 1965 PLACE, 5 3,200,010

ELECTRICAL RESISTANCE ELEMENT Filed Dec. 11. 1961 INVENTOR.

BY HIS HTTORNEYS. HARE/S, K/ECH, RusssLLac KERN THOMAS M. PLA cs, 5R.

United States Patent 0 3,2tlt),tll0 ELEQTRECAL RESESTANQE ELEMENT Thomas M. Place, Sin, Newport iii-each, Calir'., assignor to Beeltman instruments, inn, :1 corporation of Cahi'ernia Filed Dec. 11, E961, Ser. No. 158,317 3 Claims. (Cl. 117 -232) This invention relates to resistance elements and tomethods of making same and is particularly suitable for use with variable resistors and potentiometers which have a Wiper for traversing the resistance material. The invention is particularly directed to a resistance element and method of making same which eliminates the discontinuities normally encountered by a Wiper as it moves along the surface of the resistance element. The article and method of the invention is primarily intended for use in rotary potentiometers which require full 360 or continuous rotation of the wiper, but of course can be used with resistance elements of any shape or form.

The invention is intended for use with resistance elements in which the resistance material is formed as a film on a nonconductive support or base and is specifically directed to the use of resistance materials comprising fired mixtures. of glass and metal, which are often referred to as cermet materials. A number of typical cermet resistance films and methods of making same are described in US. Patents Nos. 2,950,995 and 2,950,996.

In a rotary potentiometer using a film of resistance material on a base, the annular film necessarily is formed with a gap. The terminal connections are ordinarily applied to the resistance film at each edge of the gap and one or more additional terminal taps may be utilized along the film. Ordinarily the rotation of the wiper is limited to something less than 360 by mechanical stops to avoid traversing the gap in the film. When continuous rotation of the device was desired, the Wiper would traverse the nonconductvie gap or dead space, with a number of deleterious effects. While traversing the gap, the wiper rides on the base material, typically a high-temperature resistant and electrically nonconductive fired steatite or alumina having a relatively rough and highly abrasive surface. Excessive wiper wear results from this contact and the life of the unit is materially diminished. Electrically conductive particles are abraded from the wiper by contact with the base and are imbedded in the surface of the base forming a conductive path across the gap. Contact of the. wiper with the edge of the resistance material at the gap produces wearoi the resistance material and also results in changes in the driving torque required for the potentiometer, which torque variations are significant in some applications.

Electrical connections to a resistance film may be made in various ways, such as by pressure contacts or wires alfixed to the film. The most widely used system is the application of a film of electrically conductive material onto the resistance film. The electrically conductive film, ordinarily a fired metal film, usually continues onto the base and provides a surface suitable for soldering or welding wires thereto.

Application of a terminal film onto the resistance film results in a zone of material raised above the surface of the resistance film. As the wiper moves over the terminal area, it is raised from the resistance material and then drops onto the resistance material, causing rapid degradation of the surface of the resistance material and additional wear of the wiper. Metallic particles from the terminal strip are carried into the resistance element and into the gap introducing undesirable changes in resistance of the structure.

It is an object of the invention to provide a new and improved resistance element and method of making same 32%,619 Patented Aug. 1%, 1965 which will eliminate the problems discussed above. A further object is to provide a structure incorporating a film-type resistance element with a gap and a wiper which may traverse the gap without undue wear of the wiper. A further object is to provide such a structure in which the film and gap may be traversed by a Wiper without incurring significant electrical degradation. Yet another object is to provide such a structure in which the torque or force required for driving the Wiper is not changed as the wiper traverses the terminals and gap or gaps in the resistance film.

It is a specific object of the invention to provide a resistance element and method of making same that is particularly suited for use with cermet resistance films which by nature are extremely hard and generate a number of problems with electrical noise and wiper wear.

it is an object of the invention to provide a resistance element including a resistance film for traverse by a wiper, with the resistance film having a gap therein, and a nonconductive film filling the gap and of substantially the same thickness as the resistance film to produce a smooth continuous and fiat surface for the wiper. A further object is to provide such a structure in which both the resistance film and the nonconductive film are applied and fired at one time to produce the smooth glassy continuous surface. A specific object is to provide such a structure in which both films are cermet materials with the nonconductive film including an additional component to produce the nonconductive characteristic.

It is an object of the invention to provide a resistance element including a resistance fi m and one or more terminals for electrical connection to the film with the terminals formed underneath the resistance film to provide a smooth and substantially flat surface for contact by the wiper. A further object is to provide such a structure in which the resistance film is applied as a viscous mixture over the terminal strips and then tired to produce the desired smooth and substantially flat surface. A particular object is to provide such a structure in which the terminal strip is relatively thin and the resistance film is relatively thick, with the thickness ratios being in the order of five to one or greater.

It is an object of the invention to provide a resistance element comprising an electrically nonconductive base having at least one electrically conductive terminal strip fixed thereon, a resistance film fixed thereon and overlying a portion of the terminal strip with the resistance film having a gap therein, and a nonconductive film fixed thereon in the gap with the films of substantially the same thickness and having a continuous smooth and substantially fiat surface.

It is an object of the invention to provide a resistance element comprising a high-temperature resistant, electrically nonconductive base having at least one electrically conductive terminal strip fired thereon, a cermet resistance film fired thereon and overlying a portion of the terminal strip with the resistance film having a gap therein, and a nonconductive film fired thereon in the gap and contacting the resistance film at opposite edges shows and the description merely describes preferred embodiments of the present invention which are given by Way of illustration or example. I

In the drawing: V FIG. 1 is an isometric view of a basefor a resistance element of a rotary potentiometer, with two end terminal strips and one tap terminal strip applied to the base; FIG. 2 is an isometric view of the structure of FIG. 1

with the resistance film and nonconductive film applied;-

FIG. 3 is a view similar to that of FIG. 2 showing, an alternative form of the invention.

The invention will be described hereinbelow as applied to a resistance element for a single turn rotary potentiometer. Of course, it is equally applicable to linear potentiometers and other forms of resistance elements.

The resistance element includes a support structure or base which may have the" form of an apertured disc as shown in FIG. 1. The base should be electrically nonconductive or have a nonconductive coating thereon and is preferably made of a refractory material such as steatite or alumina. In conventional practice, the ste-atite base is molded, fired, and ground or lapped to produce a smooth, fiat surface for carrying the resistance film.

Terminal strips 11, 12, 13 of electrically conductive material are applied to the base 10. The terminal strips may be applied by various of the known methods but it is preferred to use a metallic paste which is silk-screened onto the base and subsequently fired to convert the paste to a metal film.

A metallic paste may include a combination of metal and glass powders mixed with a volatile liquid carrier such as toluol, xylol, isopropyl alcohol, or even water. A suitable composition may comprise in percent by weight, silver 8.5, platinum 76.5, and lead borosilicate glass 15.0. The base with the paste terminal strips is then heated to convert the paste into a layer of metal which is firmly attached to the base. With the particular paste described above, the unit may be heated to 870 C. to 950 C. for a period of ten minutes.

A resistance film 15 is applied to the base 10 and overlies a portion of each of the terminal strips for making electrical contact with the strips. The resistance fi-lm 15 has a gap therein which ordinarily occurs between the terminal strips 11, 12, these terminal strips providing for electrical connection of the resistance element to an external circuit. The terminal strip 13 provides a tap for the resistance element and, of course, may be omitted or a number of taps may be used as desired for theparticular circuit application.

An electrically nonconductive film I6 is applied to the base filling the gap between the ends of the resistance film 15. The nonconductive fihn I6 is made substantially the same thickness as the resistance film 15 resulting in a continuous, smooth and substantially flat surface on the resistance element for contact by the wiper. Of course, the nonconductive film 16 can be applied before the resistance film 15 if desired, and either order is intended to be covered in the specification and claims.

The resistance film and the nonconductive film may be produced in any of the conventional forms and methods now in use. It is preferred to utilize the ceramic-type materials and processes described in the aforementioned U.S. Patents Nos. 2,950,995 and 2,950,- 996. A viscous cermet resistance material mixture corresponding to one of the compositions described in the aforementioned patents may be placed on the base as by silk screening. The nonconductive film 16 may then be applied in the same manner using a viscous mixture of nonconductive material. The unit is then fired as described in detail in the aforementioned patents to produce a continuous glassy and substantially flat surface on the films.

Typically, the nonconductive film may be formed of a glass which is nonconductive, such as glass A, glass 1 or glass 2 of the aforementioned patents. However, it

is preferred to form the nonconductive film of a cermet resistance material which is made nonconductive by the addition of a few percent (typically in the range of about l2 percent by weight) of another component which is relatively inert, not reacting with the glass component of either film. Suitable additives include the refractory oxides such as tin oxide, Zirconium silicate, magnesium oxide, antimony oxide, and ground fired steatite. By making the resistance film and the nonconductive film of cermets and particularly of the same cermet, diffusion of material from one film to the other is substantially eliminated. Antimony oxide is preferred as the additive since it provides a superior surface on the fired film.

A typical nonconductive cermet material may comprise in percent by weight, gold 2.50, platinum 1.67, antimony oxide 1.65, and glass A 94.l8.

The reason why inclusion of a small amount of such additional component in a cermet resistance mixture produces a nonconductive film is not fully understood. However, it is presently felt that the additional component functions in the nature of a filler, separating the conductive metal particles in the film. Another explanation being considered is that the added component causes the metal particles to gather in large lumps and thereby interrupt the conduction path in the film.

The resistance material is applied as a viscous film overlying portions of the terminal strips. While this film is relatively stiff, it flows slightly prior to and during the heating to form a smooth upper surface over the discontinuity produced by the terminal strips. Hence the resistance film will be slightly thinner where it overlies the terminal strips but the upper surface of the resistance element will be flat and no variations in contour will be encountered by the wiper as it traverses the films. The terminal strips are relatively thin in comparison with the resistance and nonconductive films. Typically a terminal strip will be in the order of 0.0001 inch thick while the films will be in the range of 0.0005 to 0.003 inch thick. Of course, the maximum firing temperature for the cermet films shouldbe less than the melting point of the terminal strips so as not to disturb the terminal strips during the subsequent firing operation.

The resistance element of FIGJZ is now ready for assembly in the usual manner'to form a potentiometer. The wiper will traverse the upper surface of the resistance and nonconductive films which present a smooth, continuous and flat surface. There are no discontinuities or bumps or changes of elevation in the surface and the wiper never contacts the base material. Hence all of the disadvantages previously discussed are eliminated.

An alternative embodiment of the invention is shown in FIG. 3. In this form, the resistance film 15 and the nonconductive film 16 are applied to the base 10 in the usual manner, as by silk screening and firing, after which terminal strips 18, 19 are applied. The method of formation of the films and strips may be the same as described in the preceding embodiment. The arrangement of FIG. 3 would primarily be used in units wherein the films were quite thick in comparison to the terminal strips so that the minor discontinuity caused by the terminal strips would not be objectionable, It is desirable, although not essential, that the terminal strips be applied over the areas where the resistance film and the nonconductive films meet. If a fired terminal strip is used, of course the firing temperature should be below that which Would produce melting of the film so as not to disturb the previously formed films.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are poss1- ble and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention: 1. A resistance element comprising an electrically nonconductive base having:

a cermet resistance film fired thereon and having a gap therein; and a nonconductive film fired thereon in said gap and contacting said resistance film at opposite edges of said gap, said nonconductive film in said gap comprising the cermet resistance material of said resistance film With up to a few percent by weight of an additive selected from the group consisting of tin oxide, zirconium silicate, magnesium oxide, antimony oxide or ground fired steatite to render said cermet material nonconductive. 2. A resistance element comprising a high temperature resistance, electrically nonconductive base:

at least one electrically conductive terminal strip fired thereon;

a cermet resistance film fired thereon and overlying a portion of said terminal strip, said resistance film having a gap therein;

and a nonconductive film fired thereon in said gap and contacting said resistance film at opposite edges of said gap, said nonconductive film being of substantially the same thickness and having a continuous, smooth and substantially fiat surface co-eXtensive with said surface of said resistance film, said nonconductive film comprising a cermet resistance material with up to a few percent by weight of antimony oxide added thereto.

3. A resistance element comprising a high temperature resistance, electrically nonconductive base having:

at least one electrically conductive terminal strip fired thereon;

a cermet resistance film fired thereon and overlying a portion of said terminal strip, said resistance film having a gap therein;

and a nonconductive film fired thereon in said gap and contacting said resistance film at opposite edges of said gap, said nonconductive film being substantially the same thickness as said resistance film and having a continuous, smooth and substantially flat surface coextensive with said surface of said resistance film, said nonconductive film in said gap comprising a cermet resistance material with up to a few percent by Weight of calcined steatite added thereto.

References Cited by the Examiner UNITED STATES PATENTS 2,796,620 5/57 Kohr-ing 29-1557 2,827,536 3/58 Moore et al 29-1557 2,848,359 8/58 Talmey 338-409 2,908,882 10/59 Gottschall 338-l31 2,935,717 5/60 Solow 338308 2,950,996 8/60 Place et al. 338308 2,994,846 8/61 Quinn 338--3 08 RICHARD M. WOOD, Primary Examiner. 

1. A RESISTANCE ELEMENT COMPRISING AN ELECTRICALLY NONCONDUCTIVE BASE HAVING: A CERMET RESISTANCE FILM FIRED THEREON AND HAVING A GAP THEREIN; AND A NONCONDUCTIVE FILM FIRED THEREON IN SAID GAP AND CONTACTING SAID RESISTANCE FILM AT OPPOSITE EDGES OF SAID GAP, SAID NONCONDUCTIVE FILM IN SAID GAP COMPRISING THE CERMET RESISTANCE MATERIAL OF SAID RESISTANCE FILM WITH UP TO A FEW PERCENT BY WEIGHT OF AND ADDITIVE SELECTED FROM THE GROUP CONSISTING OF TIN OXIDE, ZIRCONIUM SILICATE, MAGNESIUM OXIDE, ANTIMONY OXIDE OR GROUND FIRED STEATITE TO RENDER SAID CERMET MATERIAL NONCONDUCTIVE. 